1. FIELD OF THE INVENTION
This invention relates to a hard alloy containing molybdenum and a process for the production of the same and more particularly, it is concerned with a hard alloy comprising, as a predominant component, a hard phase consisting of a compound having a crystalline structure of simple hexagonal type and a process for the production of the same.
2. DESCRIPTION OF THE PRIOR ART
Up to the present time, as a starting material for cemented carbides, there has been used tungsten carbide (WC) powder as a predominant component with a suitable binder metal, typically an iron group metal, to which carbides or carbonitrides of high melting point metals such as titanium (Ti), tantalum (Ta), niobium (Nb), molybdenum (Mo), hafnium (Hf), vanadium (V) and chromium (Cr) are added depending upon the requirements of a desired alloy. However, it is also true that tungsten is a relatively expensive metal and that it is found in only a few parts of the world. Accordingly, it is considered to be a so-called "strategic" material, and its availability can be subject to political considerations. Therefore, increase of the demand for cemented carbides consisting of tungsten carbide almost inevitably presents problems of availability and if the tungsten carbide can be exchanged for another high melting point metal carbide, this exchange would have a great influence upon the industry.
Molybdenum monocarbide (MoC) is considered as a useful substitute, since this carbide only has the same crystal structure of simple hexagonal type as tungsten carbide as well as mechanical properties similar to tungsten carbide. However, the existence of the hexagonal molybdenum monocarbide as a simple substance has remained in question to this date and thus an attempt to stabilize molybdenum monocarbide has exclusively been carried out by forming a solid solution with tungsten carbide. This method was first reported by W. Dawihl in 1950, but this solid solution was not examined in detail and its commercial value was not found in those days.
Of late, however, the study to utilize the solid solution (Mo.sub.x W.sub.y)C wherein x+y=1 has become active with the rise of the price of tungsten. It is very interesting why a study on this solid solution and an attempt to use the same has not been carried out so actively up to the present time.
Molybdenum carbide is stabilized as a monocarbide having a crystal structure of simple hexagonal type when a solid solution is formed with tungsten carbide. If this stable carbide of (Mo, W)C can readily be prepared, replacement of tungsten by molybdenum would be possible. For the embodiment of this purpose, there has been proposed a process for the stable production of (Mo, W)C (Japanese Patent Application OPI No. 146306/1976- U.S. Pat. No. 4,049,380-). When the (Mo, W)C powder obtained by this process is used as a starting material of a (Mo, W)C-CO alloy as a substitute for WC, however, MoC is not stable in the alloy and Mo.sub.2 C tends to precipitate often. Furthermore, this process has not been put to practical use, since it requires a heat treatment for a long time.
Furthermore, it has been proposed to produce a molybdenum-tungsten carbonitride having a crystalline structure of tungsten carbide by heating molybdenum and tungsten in combined form and carbon in a proportion sufficient to form the monocarbide in a nitrogen-containing atmosphere (Japanese Patent Application (OPI) No. 104617/1978.). However, this method seeks to stabilize the alloy by incorporating nitrogen so that (Mo, W).sub.2 C is not precipitated.